Emergence of a new and more virulent strain of VTEC – E. coli O104
Kevin Pollock
Health Protection Scotland
The spectre of verotoxin-producing E. coli (of which E. coli O157 is the most well-known serotype) has emerged once again. Between the 1st May and 20th July of this year, an outbreak ofE. coli O104:H4 in Germany and continental Europe resulted in 3,039 cases of infection with an additional 727 cases of the haemolytic uraemic syndrome (HUS). More than 50 people died due to the infection making this the largest and most deadly VTEC outbreak ever recorded.
Aside from the magnitude and seriousness of the outbreak, it was notable for several other reasons. The infectious organism is not a typical VTEC strain. It appears to be a hybrid strain which has characteristics of VTEC such as the Shiga toxin 2 but it has also acquired genes (notably aggA) from the less pathogenic enteroaggregative E. coli, which facilitated enhanced binding of the pathogen to colonic epithelial cells. It is the property of having both these proteins, which makes this strain so virulent i.e. prolonged binding of the pathogen to cells permits the Shiga toxin to cross from the gut into the bloodstream and cause injury to the kidneys. This was evidenced in the progression rate from bloody diarrhoea and abdominal pain to the more serious HUS. E. coli O157 infections tend to progress to HUS in about 10% of cases but with this outbreak strain, the number of cases progressing to HUS was approximately 20%.
Epidemiologists were also surprised at the demographic of those being infected and hospitalised. E. coli O157 infections generally affect the young and elderly but in this outbreak, it was females in their thirties and forties who were disproportionately affected. This, coupled with the preliminary epidemiological investigations led experts to believe that women were more likely to consume salad vegetables such as cucumbers than their male counterparts. It was only when further investigations were performed such as case-control and restaurant-cohort studies, that contaminated sprouts were considered to be the likely source of infection.Trace back studies carried out by the GermanE. coli Task Force and thelocal authorities of Lower Saxony identified a single sprout-producing farm in Lower Saxony as being the most likely source of sprouts contaminated with E. coli O104. Furthermore, subsequent actions resulted in forward tracing establishing that all 41 case clusters identified at that time in Germany were linked to consumption of sprouts originating from the sprout-producing farm.
A second outbreak in France with the same strain appeared to confuse matters since nobody had consumed sprouts from the facility in Germany. All French cases reported eating raw sprouts however these were fenugreek, rocket and mustard sprouts, which had been procured from a local garden centre, supplied by a UK-based company. Therefore, the European Food Safety Authority was asked to perform a detailed trace back exercise to elucidate whether the two outbreaks shared a common source. Investigations found that an importer from Egypt was considered the most likely common link and as such, a ban was placed on further imports as well as national public health messages to thoroughly cook the aforementioned sprouts. It should be noted that microbiological confirmation of the pathogen from the suspect sprouts has yet to be proven however, this cannot be interpreted as proof that batches were not contaminated since results are limited by the analytic and diagnostic performance characteristics as well as the sampling plan. It is most likely that the contamination of fenugreek seeds happened at farm level in the country of origin i.e. Egypt.
Although the outbreak affected many countries in Europe, Scotland was largely unscathed with only one imported case who developed uncomplicated infection. However, in recent years the Scottish E. coliO157/VTEC reference laboratory andHealth Protection Scotland have detected a number of VTEC strains, which are not attributable to E. coliO157. These include the sorbitol-fermenting VTEC strain, which resulted in a nursery outbreak in 2006 [1] and the E. coli O26 strain [2], both of which resulted in serious forms of HUS in Scottish children. Elucidating the source of infection is only possible by thorough environmental health investigations into cases and in Scotland, we are blessed in having an excellent network of Environmental Health and Food Safety Officers who understand the unique epidemiology of these pathogens.Salient public health messages on simple precautionary behaviour also need to be regularly reinforced as prevention of VTEC infection prevents HUS.
[1]
[2] Pollock et al., (2011) Emergence of highly virulent E. coli O26, Scotland. Emerging Infectious Diseases, vol. 17, issue 9 (in press)